The invention relates to a graphite tube furnace for the electrothermal atomization of specimens for atomic absorption spectroscopy.
The known analysis of substances by means of atomic absorption spectroscopy is based on the recognition that any atom can absorb light of that wave form in which it itself also emits radiation. Consequently, the specimen to be analyzed, which is present in liquid or solid form, must be atomized in order to remove the atoms from a molecular association. By means of a hollow cathode lamp or the like, spectral lines are emitted which radiatively penetrate the atomic cloud of the specimen. As a result of absorption of specific spectral lines by specific atoms, the intensity of the spectral lines is attenuated. A spectral breakdown of the light in the monochromator separates out the resonance lines and displays their attenuation due to the absorption. The measure of the attenuation is a measure of the extent of the absorption and thus a measure for the presence of the quantity or of the concentration of the element under investigation in a specimen.
To atomize the specimen, the so-called graphite tube technique has become known, in which a high current flows longitudinally or transversely through a graphite tube furnace and thus the furnace is heated to a high temperature, e.g. a temperature as high as 3000.degree. C. In this case, the hollow graphite cylinder is radiatively penetrated by the light beam of the hollow cathode lamp. By way of example, 5 to 100 microliters of substance are introduced through a small aperture onto a specimen platform in the graphite tube and are heated. In this case, the graphite tube is circumcirculated by an eg. argon gas current, in order to keep the oxygen of the air away and to prevent a combustion of the graphite.
Further technical details together with further literature references are indicated in EP 0,381,948 A1, columns 1 to 4. To explain the present invention, express reference is made to this prior art.
In the aforementioned literature reference, it is pointed out that the specimen to be analyzed is in general fed onto a so-called L'vov platform. This platform--also referred to hereinbelow as the specimen take-up or specimen store--is either a separate workpiece (see, for example, DE-U1 87/13,503) or a bearing element which is also integrated into the tube when the graphite tube furnace is produced.
The treatment of the specimen which is pipetted onto the inner specimen take-up, takes place in a first drying step for concentration and in a subsequent pyrolysis step to atomize the specimen. In this case, the graphite tube furnace is heated from room temperature to over 2000.degree. C. in slightly more than one second; this takes place by very high currents within the graphite tube furnace. These high temperatures are maintained constant for a period of approximately 10 s, in order to achieve the required steps. After this, the graphite tube furnace is cooled down again to room temperature.
For the correct treatment of the specimen within these time intervals, it has already been stated in the aforementioned publication EP 0,381,948 that it is necessary to undertake a controlled heating of the graphite tube furnace and to avoid a premature atomization of the specimen. Accordingly, the heating process, the drying process and the subsequent atomization process should proceed with very great precision in terms of time, in order that, especially for the atomization process, a strong absorption signal should be obtained. In order to achieve this, according to the aforementioned publication it was proposed that the specimen take-up, having the shape of a semicylindrical shell, is connected as an inner body only at one end with the outer tube, so that on the one hand no current flows through said specimen take-up and thus it is not additionally heated. On the other hand, the intention is to avoid to a large extent a conduction of heat from the tubular furnace body through which current flows, to the specimen take-up via the connecting web. The heating of the specimen is to take place as far as possible exclusively by radiant energy via the tubular furnace body.
Since the tubular furnace body is intensely heated directly by the passage of a high current, an intense radiation also becomes established, which acts directly on the specimen lying on the open specimen take-up. In this case, the radiant energy or radiant power given off to the specimen is proportional to the fourth power of the radiation temperature, i.e. the radiant power acts directly and lastingly on the specimen to be analyzed. Thus, the radiation acting directly on the specimen prevents an effective delay of the atomization of the specimen. Indeed, DE 2,554,950 C2 discloses a graphite tube for the atomization of specimens, in which tube the inner body is designed as a cylindrical tube to take up the specimen. This inner tube is, however, connected to the likewise tubular outer body by a plurality of longitudinally extending ribs, so that this inner body having the shape of a cylindrical shell, through which a current likewise flows, is subjected to an intense heating, which acts directly on the specimen by the giving-off of radiant energy.